Not Applicable
1. Field of the Invention
The present invention generally relates to the field of liquid crystal display (LCD) manufacturing. More particularly, the present invention relates to a process and means for increasing the viewing angle of a liquid crystal display by modifying the alignment layer.
2. Description of the Related Art
Liquid crystal displays generally have an alignment layer, which is treated or processed prior to coming in contact with a liquid crystal to produce a preferred direction along which the liquid crystal becomes oriented. Presently, the display industry uses mostly polyimide which is rubbed by a scanning rotating roller to which is attached a special cylindrically draped fibrous cloth. The cloth in turn is in contact with the alignment layer to impart a uniform directionality in the polyimide. The rubbing process physically changes the structure of the polyimide to orient the liquid crystal director vector to position itself in a single uniform direction along the polyimide alignment layer.
Recently, films of diamond-like carbon (DLC) have been used as the alignment layer in place of polyimide as described in U.S. Pat. No. 6,020,946 entitled xe2x80x9cDry processing for liquid-crystal displays using low energy ion bombardmentxe2x80x9d with inventors Callegari et al., issued Feb. 1, 2000, and is hereby incorporated by reference in its entirety. A scanning ion beam is used to produce the required directionality or orientation of the DLC alignment layer for aligning the liquid crystal. The ion beam is scanned over the diamond-like carbon layer at a beam angle less than 90 degrees with respect to the planar surface of the alignment layer. This type of scanning imparts a preferred direction for the liquid crystal orientation when in contact with the ion beam treated DLC. This method for producing LCD""s although useful is not without its shortcoming. One shortcoming is a limited viewing angle of the display. The viewing angle is defined as the angle subtended by the line of sight of the viewer and the normal to the display screen. Accordingly, a need exists to overcome the limited viewing angle shortcoming for LCD""s.
Moreover, for both the polyimide and the diamond-like carbon, a twisted nematic type of liquid crystal material has a limited viewing angle when used in a liquid crystal display due in part to the uniform or single domain alignment direction that results from the conventional rubbing of the polyimide or the ion beam scan for DLC. Accordingly, a need exists to overcome the limited viewing angle shortcoming for both polymide and diamond-like carbon LCD""s as well.
Generally, an alignment layer is used to orient the liquid crystal along a desired direction. For the use of twisted nematic liquid crystal material, there is conventionally a single direction along which the crystal becomes aligned. This configuration, however, suffers from a relatively small viewing angle, that is when the screen is viewed at an angle other than near normal incidence to the screen, the light intensity of the display rapidly decreases. The present invention describes a method for increasing the viewing angle using a diamond-like carbon (DLC) alignment layer or a polyimide alignment layer irradiated with a scanned ion beam to produce more than one direction of liquid crystal alignment. The new process gives rise to a dual-domain structure in the alignment layer instead of the customary single domain. The dual-domain structure greatly enhances the viewing angle of the display.
The increase in intensity enhancement with viewing angle is made possible by creating domains of different directions in the alignment layer. A multi-domain alignment film is one where the liquid crystal will have more than one orientation direction when the display panel is not receiving electrical signals. For example, in a typical dual domain alignment layer, the two liquid crystal directors, that is the vectors that define the orientation direction of the liquid crystal in contact with the alignment layer will lie nearly 180 degrees apart with respect to one another. The angle between the two directions will be (180xc2x0xe2x88x922xcex2), where xcex2 is the angle, usually a few degrees, the director makes with respect to the plane of the alignment layer. This angle xcex2 is also known in the art as the pre-tilt angle.
The present invention uses an ion beam to scan over a pre-deposited layer of diamond-like carbon (DLC) or other films that can be physically altered by scanning with an ion beam at angle Ø, where Ø is the angle subtended between the direction of the ion beam and the planar surface of the alignment layer. In present day practice a display panel consists in part of a substrate onto which thin film transistors have been deposited. The transistors function to selectively charge and discharge small thin film capacitor plates. Generally these plates are transparent to optical radiation. A second capacitor plate is deposited on a second substrate. It is located in such a manner that when the two substrates are appropriately affixed to one another, the two corresponding plates form a two-plate capacitor, separated from one another by several microns of liquid crystal filling the space between them. The liquid crystal serves as the dielectric of the capacitor. Typical thin film capacitor plates in liquid crystal displays consist of indium tin oxide (ITO), an electrically conducting but optically transparent material. Each capacitor plate on the substrate onto which the alignment layer is deposited is surrounded along in its periphery by gate and data lines which impart the desired electrical bias to the thin film transistor and hence to the corresponding capacitor plate. The voltages impressed on the data/gate lines determine the voltage across the capacitor. This applied voltage will cause a certain amount of rotation of the crystal material between the plates. This acts as a type of window shade which depending on the amount of rotation will allow more or less of the back lighting to pass through.
The combination of liquid crystal, alignment layer, gate/data lines, transistor and thin film capacitor plate constitute a pixel. The thin film capacitor plate is referred to as a pixel electrode. A display panel may have several thousand or even several million independently controlled pixels. A column of pixels is made up of the pixels positioned between two adjacent data lines while a row of pixels consists of pixels positioned between two adjacent gate lines.
To provide a uniform direction for the liquid crystal alignment, an argon ion beam is used for scanning the alignment layer. In other embodiments, ions such as helium, nitrogen, neon, krypton, and xenon are used. The ion beam is partially electrically neutralized by injecting electrons after the ions have been accelerated to the desired energy level through a series of accelerator grids. Thus, the scanning ion beam is very nearly electrically neutral so that there is negligible net charge buildup on the alignment layer. The scanning ion beam has recently been utilized to obtain a unidirectional alignment in DLC films i.e., a single domain orientation for the liquid crystal. It has not been previously disclosed how the ion beam can be utilized to achieve more than a single domain, that is to provide multi-domain alignment conditions. To obtain a dual domain alignment layer as the present invention describes, two separate ion beam scans are required. The first scan is made with no potential impressed on the gate/data lines. This is followed by a second ion beam scan during which the data/gate line voltages cause the thin film transistors to bias alternate rows or columns of thin film capacitors respectively to either positive or negative potentials or to positive and near zero potentials. The second scan over the alignment layer is usually, though not necessarily, made from the opposite direction of the first scan. The result of the second scan is one in which the positively biased pixel or capacitance electrodes repel the oncoming positively charged ions by way of electrostatic repulsion. The alignment layer over the negatively or neutrally biased capacitor electrodes will be reversed or re-written in a direction opposite to the direction resulting from the first ion beam scan. Using this dual scan technique, alternate rows or columns of pixels will have opposite alignment directions. It is also possible to make two or more successive rows or columns of pixels have the same alignment with the next two or more rows or columns with opposite alignment directions by choosing appropriate data/gate line voltages for the respective rows and columns.